1. Field of the Invention
This invention relates to tape storage media and more particularly relates to apparatuses, systems, and methods for managing data storage on segmented tape media to maximize use of available storage capacity and to optimize fast access to stored data.
2. Description of the Related Art
With each new generation of tape storage technology, the capacity of available tape storage products increases in response to the general demand for greater storage capabilities. The total storage capacity of a tape depends on many factors, including the physical dimensions of the tape, the data compression, if any, used to write data to the tape, the number of tracks across the width of the tape, and so forth. Furthermore, the usable storage capacity is often defined to be slightly less than the physical capacity of the tape media. This is due, in part, to servo tracks, data block headers, and other metadata blocks existing throughout the tape.
For tape storage applications, a tape drive typically enables data to be stored onto a magnetic tape medium, such as a metallic ribbon, within a tape cartridge. The tape medium conventionally is designed to include a plurality of tracks that are distributed across the physical width of the tape medium and run the physical length of the tape medium. A tape write head within the tape drive is typically capable of writing up to about sixteen tracks at one time, starting at one end of the tape and moving along the length of the tape. When the tape write head reaches the end of the tape, the head is aligned over the proximate track set, the direction of the tape is reversed, and the write head writes an additional sixteen tracks in the opposite direction. This “serpentine” pattern may continue until all tracks have been written.
The process for reading data from the tape medium is essentially the same. A tape read head moves across the tape medium and reads sixteen tracks from one end of the tape medium to the other. The tape read head then realigns to read an additional sixteen tracks and moves over the second set of tracks in the opposite direction.
Given the large capacity of conventional tape storage devices, various data blocks may be stored on a single tape medium. The location of each of these data blocks may be marked on the tape using block header information, data pads (areas of tape where data is intentionally not written), and other conventional identification means and methods. The tape read head is able to locate a particular block of data by using one or more servo tracks that are written onto the tape storage medium.
As the total tape storage capacity increases, however, so too does the time required to access data on the tape media. Because of the physical length of the tape and the increased number of bits on the tape, the amount of time required to fulfill a read request typically varies depending on the location of the data stored on the tape. Faster data access is generally available for data stored near the front of the tape, while data stored near the end of the tape typically requires a longer access time to scroll through the length of the tape.
In certain time-sensitive applications, delayed data access can cause negative effects and may impede subsequent read requests or other processes. These effects can cause Service Level Agreements to be missed. However, for many other data storage uses, such as data backup, for example, the infrequent need to access that data makes longer access times generally acceptable.
More recently, manufacturers of tape storage products have directed their attention in part to improving data access time using tape storage drives and cartridges. One method to address the problem of increased data access time employs various levels of transparent buffering in which tape data may be stored in connection with other storage mediums, such as a direct access storage device (DASD) or an optical disk.
If the requested tape data is stored on a DASD cache as with a virtual tape subsystem, data retrieval time may be improved greatly. However, the storage capacity of a DASD cache is typically significantly less than that of a tape storage system. Currently, technology allows about five hundred Gigabytes (GB) of non-compacted data to be stored on a single, standard tape cartridge. The previous generation of tape media provided for a storage capacity of about three hundred GB. In addition, the DASD cache must migrate much of the data to tape cartridges in order to be able to buffer more recent data. For this reason, a DASD cache only improves data retrieval time for the data that is in the DASD cache at the time of the data request, but does not improve the access time for the large portion of data that has been migrated to tape and demoted from the cache.
Another known method of decreasing data access time is to segment the tape storage medium into two or more segments and to write data to the segments in a sequential manner. A tape segment may include a specified storage capacity, or a physical length of tape, that is less than the total capacity of the tape. For example, a tape storage medium may be divided into two segments. When writing data to the tape, the data is written to the first segment, which may facilitate fast access, and then to the second segment, which typically requires slower access. This method improves data access time in that the first segment may be written to or read from without physically forwarding all the way to the end of the tape storage medium. Data access time is greatly improved when the first segment is located at the load point of the tape.
The first segment may include, for example, about one fifth of the total tape storage capacity. Thus, the tape drive need only advance one fifth of the way through the total length of the tape medium, rather than all the way to the physical end of the tape, before reversing direction. The data stored on the first segment generally can be accessed quicker than data distributed along the entire length of the tape.
In certain embodiments, the tape is ended after the first segment is filled. However, limiting the storage capacity of the tape negates one of the greatest advantages provided by tape storage media. Providing segmented tape media optimizes the data storage capacity and yet at the same time allows fast access for retrieving data in the first segment.
When storing data to the entire length of the segmented tape, determining or selecting data for fast access can be problematic and even counterproductive in certain instances. Because data can be written to the second segment of the tape only after the first segment of the tape has been filled, storage to the fast access portion of the tape must be carefully monitored to ensure that slower access data is not stored to the fast access portion. In addition, accessing data stored on the first segment of the tape may be hindered or slowed by a read request for data stored on the second segment of the tape, thereby minimizing the advantages of providing fast access storage.
From the foregoing discussion, it should be apparent that a need exists for an apparatus, system, and method that enables fast access to data stored on tape media while further enabling utilization of the large storage capacity of the tape. Beneficially, such an apparatus, system, and method would determine whether data requires fast access and would store the selected data on the fast access portion of the tape media. Furthermore, the apparatus, system, and method would enable storage on the slower access portion of the tape media for data storage uses not requiring fast access.